This invention relates generally to neutral beam injection systems for use in controlled fusion devices such as tokamaks, magnetic mirror systems, bumpy tori and the like, and more particularly, this invention relates to improvements in neutral beam injectors with direct ion energy recovery.
In the field of controlled fusion or controlled thermonuclear reaction, a high temperature plasma is formed of fusionable light isotope ions contained within a magnetic field confinement or containment zone, in an evacuated region. Such light isotopic species may generally comprise one or more materials, such as hydrogen, deuterium, tritium, helium 3, etc., which undergo fusion reactions under appropriate conditions of time, density and temperature. These conditions may be brought about or supplemented by the injection of properly accelerated neutral particle beams of one or more of the appropriate species in the proper trajectory with respect to the magnetic field. A portion of the energetic neutral particles are ionized either by collision with other neutral or charged particles or by action of the magnetic confinement field (Lorentz force) and are accordingly trapped to form a high temperature plasma in the magnetic containment zone. The injected particles must be neutral in order to penetrate the very strong magnetic field containing the plasma.
Since the neutral particles cannot be directly accelerated to high velocities, i.e., high kinetic energies, they are produced in an indirect manner from an ion source. It has been the practice to produce a neutral beam by accelerating either positively or negatively charged ions of one or more of the above appropriate species emerging from an ion source through a gas cell neutralizer wherein they interact with neutral atoms of the same species at a specified pressure through charge exchange. A portion of the ions are neutralized and emerge from the neutralizer as high energy neutral particles along with the accelerated beam passing therethrough. The beams are generated and manipulated in a vacuum chamber whose pressure is maintained at the selected pressure level by a cryopumping system.
Since the beam emerging from the neutralizer also contains electrons and unneutralized ions, some means must be provided to separate the neutral particles from the electrons and ions to obtain the desired neutral beam for injection into the magnetically confined plasma. This is accomplished by either electrostatic or magnetic field blocking of the electrons and diversion or bending of the positive charged ions from the primary beam path direction to allow the neutral beam to continue along the accelerated beam path. Depending upon the species and energy of the initially ionized particles of the beam, the neutral particle beam emerging from the neutralizer contains a large proportion of high energy unneutralized ions. Present ion sources operating at energies of about 40 kilo-electron volt (keV) per nucleon at 60 amps ion current provide about a 60% conversion efficiency in the neutralizer cell. As future ion sources are developed toward energies of about 100 keV, or higher, per nucleon, at comparable current levels, the conversion efficiency will drop and may drop to about 15% for H.degree. neutrals in H.sub.2 gas and about 45% for D.degree. neutrals in D.sub.2 gas, which represents an intolerable energy loss.
Therefore, in order to produce neutral beams for fusion plasma heating efficiently, the energy contained in the unneutralized fraction of the beam must be recovered. In order to recover the kinetic energy of the charged ion component of the beam emerging from the neutralizer cell, in the form of usable electric energy, the electrons present in the beam must be blocked and the beam ions diverted from the neutral beam line, decelerated and collected. The electrons must be blocked from entering the ion collector since they would be accelerated into the ion collector thereby producing an energy loss which may be equal to or greater than the recovered ion energy.
In the process of developing direct energy recovery in neutral beam injectors, various means have been devised or suggested, which may be generally divided into two groups, depending upon the ion deflection method used. There are either electrostatic or magnetic ion deflection methods.
An electrostatic deflection system is described in "Proceedings of 7th Symposium on Engineering Problems of Fusion Research," 1978, By W. L. Barr et al, Vol. I, p. 308. This paper discloses an electrostatic system developed at Lawrence Livermore Laboratory, Livermore, California, in which the neutralizer cell wall is held at ground potential, the ion beam collector is biased highly positive (approx. 1000 kv) and the electrons emerging from the cell are repelled by a negative voltage (approx. 20 kv) applied to electrodes which closely encompass the beam. One negative electrode is placed between the neutralizer cell exit and a generally funnel-shaped ion collector which also encompasses the beam. The other negative electrode is placed at the exit of the collector. The ion collector acts to decelerate and collect the ions diverging radially from the beam. The negative electrodes in this system must be biased sufficiently negative to drive the beam potential negative even on the axis in the presence of the positive-ion space charge and the nearby positive-ion collector. There are inherent problems with this system which include severe gas-pressure requirements for efficient direct conversion, increased beam line length in order to establish the retarding electric field which consequently reduces the neutral power transmission efficiency and the need to hold a high positive potential on the ion collector in the presence of spatial and time varying magnetic fields. The most critical gas-pressure requirement placed on this direct conversion system is imposed by the power load resulting from the acceleration and collection of the slow ions and electrons produced by ionization and charge exchange of the background gas. The resulting emission of secondary electrons at negative high voltage and the subsequent power drain must also be considered.
Other electrostatic electron-blocking and ion-deflection systems utilizing electrostatic grids which intercept the beam are discussed by P. Raimbault in EUR-CEA-FC-823, 1976. One specific system outlined in this reference employs a cylindrical grid arrangement which surrounds the beam exiting the neutralizer which is biased negative with respect to the neutralizer to suppress the electrons. The ion collection method of this system has one advantage and one disadvantage compared to the Barr system mentioned above. The single advantage is that the ion collector is at ground potential. However, in addition to the other disadvantages to the Barr system, the Raimbault system also suffers from direct interception of the ion beam on the high negative potential, cylindrial grid. Not only is the ion energy lost, but secondary electrons ejected from the grid by the ion impingement constitute an additional power loss. In the proposed Raimbault system, the ion source is operated at near ground potential and the ions are accelerated by operating the neutralizer at a high negative potential. The positive potential, V.sub.R, at which the ion source is held above ground potential is necessary to ensure that the unneutralized ions are able to reach the ion collector plate.
By operating the neutralizer at a negative potential to accelerate the ions from the source makes it possible to recover the energy of the ions at ground potential and eliminates the problems associated with a high positive potential deceleration voltage on the ion collector for recovery of their kinetic energy directly.
Further, as pointed out above, it has been suggested in the art to employ magnetic means for deflecting the ions from the accelerated beam and it has been further suggested to employ magnetic suppression, or blocking, of the electrons from the beam emerging from the neutralizer tube. It has been recognized in the art that magnetic suppression would be advantageous in that the magnetic field can penetrate beams that are too thick and too dense for electrostatic suppression to work. However, in the prior art experiments employing magnetic suppression, electrostatic fields also present in the system from positive potential deceleration ion collectors, have produced unnecessarily long beam lines and/or complicated electron motions which produce long-lived electrons in the system some of which cause unwanted power drain or which tend either to reionize the neutral beam exiting the neutralizer cell or deionize the positive ions directed to the energy recovery ion collectors.
Therefore, it will be appreciated that there is a need for a workable system for a neutral beamline with ion recovery based on the advantages of magnetic blocking of electrons and beam ion deflection.